Method of evaluating initial parameters and target values for feedback control loop of wavelength tunable system

ABSTRACT

A method of determining initial parameters and target values for tuning an emission wavelength of a wavelength tunable laser capable of emitting laser light in a substantial wavelength range is disclosed. The method iterates an evaluation of initial parameters and target values at target wavelengths in a preset order. The evaluation includes steps of supplying empirically obtained parameters to the t-LD, confirming whether the t-LD generates an optical beams, determining the initial parameters and the target values by carrying out feedback loops of the AFC and the APC when the t-LD generates the optical beam, or shifting the wavelength range so as to exclude the current target wavelength when the t-LD generates no optical beam.

CROSS REFERENCE TO RELATED APPLICATION

the present application is based upon and claims the benefit of priorityunder 35 USC 119 of Japanese Patent Application No. 2016-137116 filed onJul. 11, 2016, the entire disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wavelength tunable laser system and amethod of tuning the wavelength thereof.

Related Background Arts

A wavelength tunable laser and a system implementing such a wavelengthtunable laser are known in the field. For instance, a Japanese PatentApplication laid open No. H06-112570A has disclosed such an apparatus. Awavelength tunable laser (t-LD) in the oscillation wavelength thereofmay be tuned by parameters stored in a memory, where the parameters areevaluated by practically driving the t-LD in advance to a delivery ofthe t-LD. A t-LD is generally requested to be operable in a wavelengthrange set by the specification or by requests in the field. Accordingly,the evaluation of the parameters is first performed in an intermediatewavelength within the wavelength range, then, expanded toward shorterand longer wavelengths. However, a t-LD sometimes is unable to oscillateat wavelengths border of the wavelength range. Or, when the evaluationof the parameters is first performed in one of borders in the wavelengthrange then expanded toward another border, a t-LD sometimes is unable tooscillate in wavelengths close to the another border. In those cases,the parameters already obtained become waste.

SUMMARY OF INVENTION

One aspect of a present invention relates to a method of determininginitial parameters and target values for feedback loops of an auto-powercontrol (APC) and an auto-frequency control (AFC) that tunes awavelength and adjust power of an optical beam output from a wavelengthtunable laser (t-LD). The t-LD of the present invention is operable at aplurality of target wavelengths within the wavelength range andattributed to an optical gain with a maximum in a center of thewavelength range bur gradually decreasing in peripheries therein. Themethod of the invention includes: iterating an evaluation of initialparameters and target values at target wavelengths by a preset order,where the evaluation includes steps of, (a) supplying parameters to thet-LD where the parameters are empirically obtained; (b) confirmingwhether the t-LD generates an optical beam with a wavelength around acurrent target wavelength or not; (c-1) when the t-LD generates theoptical beam, performing feedback loops of the AFC and the APC fordetermining the initial parameters and the target values at therespective target wavelengths, and (d-1) storing the initial parametersand the target values in a memory as linking with the targetwavelengths, or (c-2) when the t-LD generates no optical beam orunstably operates, shifting the wavelength range so as to exclude thecurrent target wavelength. A feature of the method of the presentinvention is that the preset order is alternately arranged between ashortest target wavelength and a longest target wavelength at which theevaluations are not performed yet.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 shows a functional block diagram of a wavelength tunable systemaccording to the present invention;

FIG. 2 shows a cross section of a wavelength tunable laser (t-LD)implemented within the wavelength tunable system shown in FIG. 1;

FIG. 3 shows typical transmittance of etalon;

FIG. 4 shows an example of an order for evaluating initial parametersand target values for driving the t-LD, which is comparable to thepresent invention;

FIGS. 5A and 5B show wavelength dependence of magnitudes of an opticalbeam output from the t-LD and an optical gain attributed to the t-LD;

FIGS. 6A and 6B show another wavelength dependence of magnitudes of anoptical beam output from the t-LD and an optical gain attributed to thet-LD;

FIG. 7 shows a flow chart for evaluating the initial parameters and thetarget values for driving the t-LD at respective wavelengths;

FIGS. 8A and 8B schematically show an order for evaluating the initialparameters and the target values for driving the t-LD according theembodiment of the present invention; and

FIG. 9 shows a cross section of another t-LD according to the secondembodiment of the invention.

DESCRIPTION OF EMBODIMENT

Next, embodiment according to the present invention will be described asreferring to accompany drawings. The present invention is not restrictedto the embodiment, and has a scope defined in claims and everymodification within the claims and equivalents thereto. Also, in thedescription of the drawings, numerals or symbols same with or similar toeach other will refer to elements same with or similar to each otherwithout duplicating explanations.

First Embodiment

FIG. 1 shows a functional block diagram of an apparatus of a wavelengthtunable laser system 100 that includes a wavelength tunable laser (t-LD)30 that integrates a semiconductor optical amplifier (SOA) that operatesas a power controller of the wavelength tunable laser system 100. TheSOA may adjust the magnitude of an optical beam output from the t-LD 30but fully cut the output power of the t-LD 30. The wavelength tunablesystem 100 may further include a wavelength locker 50, a memory 60, anda controller 70 that integrates another memory type of a random accessmemory (RAM) therein.

FIG. 2 shows a cross section of the t-LD 30, where the t-LD 30 includesa sampled grating distributed feedback (SG-DFB) region A, a chirpedsampled grating distributed Bragg reflector (CSG-DBR) B, and asemiconductor optical amplifier (SOA) C, where the arrangement of thet-LD 30 of the present embodiment may be regarded as a semiconductorlaser including a wavelength selective mirror within the semiconductorstructure.

The CSG-DBR region B, the SG-DFB region A, and the SOA region C arearranged in this order from a front side toward a rear side. The SG-DFBregion A has an optical gain and a sampled grating that includes gratingregions 18 and space regions alternately arranged to each other alongthe optical axis of the t-LD 30. The SG-DFB region A and the CSG-DFBregion B constitute a laser region 30 a, while, the SOA region Ccorresponds to the SOA region 30 b each shown in FIG. 1.

The SG-DFB region A includes a lower cladding layer 2, an active layer3, an upper cladding layer 6, a contact layer 7, and an electrode 8stacked on a substrate 1 in this order. The CSG-DBR region B includes,also on the substrate 1, the lower cladding layer 2, a waveguide layer4, the upper cladding layer 6, an insulating layer 9, and some heaters10, stacked also on the substrate 1 in this order. The SOA region Cincludes the lower cladding layer 2, an optical amplifying layer 19, theupper cladding layer 6, a contact layer 20, and an electrode 21 alsostacked on the substrate 1 in this order. The lower cladding layer 2,and the upper cladding layer 6, extend in all regions A to C; while, theactive layer 3, the waveguide layer 4, and the optical amplifying layer19 are specific to the respective regions, A to C, but lower interfacesthereof against the top of the lower cladding layer 2 are leveled in allregions, A to C. Also, an interface between the SG-DFB region A and theCSG-DBR region B corresponds to an interface between the active layer 3and the waveguide layer 4.

The SOA region C has a facet, a front facet, covered with a film 16 thatextends from the substrate 1 to the upper cladding layer 6. The film 16is a type of, what is called as an anti-reflection (AR) film. TheCSG-DBR region B provides another facet, a rear facet, also covered witha film 17 that extends from the substrate 1 to the upper cladding layer6. The other film 17 is also a type of the anti-reflection (AR) film.The substrate 1, the lower cladding layer 2, and the upper claddinglayer 6 may be made of n-type InP, n-type InP and p-type InP,respectively. The lower cladding layer 2 and the upper cladding layer 6show a function of optical confinement for the active layer 3, thewavelength layer 4, and the optical amplifying layer 10.

The active layer 3, which may be made of semiconductor material havingan optical gain, has a quantum well structure comprising a plurality ofwell layers each made of Ga_(0.32)In_(0.68)As_(0.92)P_(0.53) with athickness of 5 nm and a plurality of barrier layers each made ofGa_(0.22)In_(0.78)As_(0.47)P_(0.53) with a thickness of 10 nm, where thewell layers and the barrier layers are alternately stacked to eachother. The waveguide layer 4 may be made of bulk material, for instance,bulk Ga_(0.22)In_(0.78)As_(0.47)P_(0.53), with bandgap energy greaterthan that of the active layer 3.

The optical amplifying layer 19 has an optical gain by supplying a biascurrent through the electrode 21 and shows a function of the opticalamplification. The optical amplifying layer 19 may also have the quantumwell structure comprising a plurality of well layers made ofGa_(0.35)In_(0.65)As_(0.99)P_(0.01) with a thickness of 5 nm and aplurality of barrier layers made of Ga_(0.15)In_(0.85)As_(0.32)P_(0.68)with a thickness of 10 nm, where the well layers and the barrier layersare alternately stacked to each other. In an alternative, the opticalamplifying layer 19 may be a bulk material made ofGa_(0.44)In_(0.56)As_(0.95)P_(0.05).

The contact layers, 7 and 20, may be made of p-typeGa_(0.47)In_(0.53)As. The insulating layer 9 is made of silicon nitride(SiN) or silicon oxide (SiO₂). The heaters 10 are a thin film resistormade of composite metal of titanium tungsten (TiW). The heaters 10 eachmay extend several segments in the CSG-DBR region, where one segment isconsisted of one grating region and one space region neighbor to thegrating region.

The electrodes, 8 and 21, and electrodes, 11 and 12, may contain gold(Au). Also, the substrate 1 provides a back metal 15 in a bottom surfacethereof opposite to a surface on which the lower cladding layer 2 isformed. The back metal 15 extends in the whole regions, A to C.

The facet films, 16 and 17, are each an AR film having reflectivity lessthan 1.0% viewed from the SOA region C and the CSG-DBR region B,respectively, which means that the facet films, 16 and 17, cause noreflection for light subject to the t-LD 30. The facet films, 16 and 17,are made of, for instance, magnesium fluoride (MgF₂) and/or titaniumoxide (TiO₂). In an alternative, a t-LD may have the facet films havingsubstantial reflectivity. For instance, a t-LD providing an opticalabsorption region in an outer side of the CSG-DBR region B, a facet film17 with substantial reflectivity may suppress light leaking out from thefacet film 17. The substantial reflectivity is, for instance, greaterthan 10%.

The grating regions 18 exist in the SG-DFB region A and the CSG-DBRregion B putting the space regions therebetween. Such an arrangement ofthe grating regions 18 and the space regions is called as the sampledgrating. One grating region and one space region neighbor to the onegrating region constitutes one segment in the lower cladding region 2.In other words, one space region sandwiched by the grating regions andone of grating regions putting the space region therebetween constitutesthe segment. The grating regions 18 each include periodicallydistributed corrugations made of semiconductor material havingrefractive index different from that of the lower cladding layer 2. Whenthe lower cladding layer is made of InP, the periodically distributedcorrugation may be made of, for instance,Ga_(0.22)In_(0.78)As_(0.47)P_(0.53). The reason why the periodicallydistributed regions are called as the corrugations is that, during aprocess of forming the grating regions 18, the lower cladding layer 2 ina surface thereof is etched to form a corrugation, and this corrugationis to be buried with a semiconductor material of quaternary compound ofGa_(0.22)In_(0.78)As_(0.47)P_(0.52). Thus, the grating regions 18 mayshow traces of the corrugation.

The grating region 18 may be formed by, for instance, two beamsinterference exposure technique, while, the space regions between thegrating regions 18 may be formed by double exposure technique, that is,after the two beams interference exposure for the grating regions, onlythe space regions are exposed to beams. The grating regions 18 in theSG-DFB region A in the corrugations thereof may have a pitch same with apitch of the corrugations in the grating regions in the CSG-DBR regionB, or the corrugations in the grating regions 18 may have respectivepitches different from each other. Also, the grating regions 18 in therespective segments may have a length along the optical axis common tothe respective grating regions 18, or respective lengths difference fromeach other. In an alternative, the grating regions 18 in the SG-DFBregion A may have a length common to the respective regions 18 and thegrating regions 18 in the CSG-FOR region B may have another lengthcommon to the respective regions 18, however, the former length in theSG-DFB region A is difference from the length in the CSG-DFB region B.

The segments in the SG-DFB region A have an optical length thereofsubstantially equal to each other, while, at least two segments in theCSG-DBR region B have optical lengths different from each other.Accordingly, an average of the optical lengths of the segments in theSG-DFB region A is different from an average of the optical lengths ofthe segments in the CSG-DBR region B. Thus, the segments in the SG-DFBregion A and the segments in the CSG-DBR region B constitute a cavity inthe t-LD 30.

As described, the SG-DFB region A and the CSG-DBR region B constitutethe laser cavity within which light reflected in the SG-DFB region A andthe CSG-DBR region B interferes. The SG-DFB region A provides the activelayer 3, which inherently shows an optical gain spectrum having aplurality of peaks with even magnitudes and a specific period toneighbor peaks. On the other hand, the CSG-DBR region B causes areflection spectrum with a plurality of peaks each having magnitudesdifferent from each other but a specific period to neighbor peaks, wherethe period between the gain peaks attributed to the SG-DFB region A isdifferent from the period between the reflection peaks attributed to theCSG-DBR region B. Accordingly, the t-LD 30 may oscillate at a wavelengthat which one of the gain peaks in the SG-DFB region A coincides with oneof the reflection peaks attributed to the CSG-DBR region B, and thewavelength at which the laser emission occurs may be tuned by varyingthe period and the wavelengths of the gain peaks and the period and thewavelengths of the reflection peaks.

As FIG. 1 indicates, the t-LD 30 is mounted on the firstthermos-electric controller (TEC) 31 that includes a Peltier Elements.The TEC 31 may operate as or show a function of a temperaturecontroller. The first thermistor 32 is also mounted on the first TEC 31,which senses a temperature of the TEC 31. The t-LD 30 in a temperaturethereof may be estimated through the sensed result of the firstthermistor 32.

The wavelength locker 50 includes beam splitters (BS), 51 and 52, firstand second photodiodes (PDs), 53 and 56, an etalon 54, a second TEC 55,and a second thermistor 51. The BS 51 is set on a position where the BS51 may split a beam coming from the front facet of the t-LD 30, while,the other BS 52 is set on a position where the BS 52 may split one ofbeams split by the former BS 51. The first PD 53 is set at a potion todetect one of the beams split by the latter BS 52. The etalon 54inherently has periodical transmittance for light entering therein. Theembodiment may implement, what is called, a solid etalon as the etalon54. The solid etalon has the transmittance whose period depends on atemperature thereof. Accordingly, the etalon 54 of the presentembodiment is set at a positon to transmit one of the beams split by thesecond BS 52 an on the second TEC 55 that may include Peltier elementsand operate as a temperature controller. The second PD 56 is set at aposition where the second PD may detect a beam passing the etalon 54.The second thermistor 57, which is mounted on the second TEC 55, maycontrol or vary a temperature of the etalon 54.

The memory 60 is a type of rewritable memory, typically, a flash memory.The controller 70 implements a central processing unit (CPU), randomaccess memories (RAMs) 70 a, a power supply, and so on. The RAMs 70 amay store programs the CPU executes and data the CPU referrers to. Thememory 60 may hold parameters for initial operations of the wavelengthtunable laser system 100 and/or target values for feedback operationseach linked with respective channels or wavelengths, which are sometimescalled as target wavelengths. For instance, the target wavelengthscorrespond to wavelength grids determined by an internationalorganization called as ITU-T (International Telecommunication UnionTelecommunication Standardization Sector). The specification hereinbelow sometimes calls the wavelengths each corresponding to thewavelength grids above as the grid wavelengths.

Table below exemplarily lists initial parameters and target values to beheld within the memory 60. The initial parameters are a bias currentI_(LD) supplied to the electrode 8 in the SG-DFB region A, a biascurrent I_(SOA) supplied to the electrode 21 in the SOA region C, atemperature T_(LD) set in the TEC 31 for the t-LD 30, and power,P_(Heater1) to P_(Heater3), supplied to the respective heaters 10 in theCSG-DBR region B; while, the target values of the feedback controlincludes a photocurrent I_(m1) output from the first PD 53 and a ratioI_(m2)/I_(m1) of the photocurrents, that is, an output from the secondPD 56 against that from the first PD 53. These initial parameters andtarget values are prepared for the respective wavelength grids andevaluated using the wavelength monitor 80 in advance to the delivery ofthe wavelength tunable system.

TABLE I initial parameters target values I_(LD) I_(SOA) T_(LD)P_(Heater1) P_(Heater2) P_(Heater3) I_(m1) Grid [mA] [mA] [° C.] [mW][mW] [mW] [μA] I_(m2)/I_(m1) 1 150.0 67.4 52.5 29.4 57.5 50.7 315 1.18 2150.0 47.7 34.5 64.4 81.3 72.5 317 1.52 3 150.0 50.9 38.7 59.1 77.7 69.1313 1.23 | | | | | | | | | n 150.0 54.8 54.0 41.2 43.3 11.7 317 1.44

Next, an operation of the wavelength tunable system will be described asreferring to FIGS. 1 and 2. An external system notifies the targetwavelength, namely, one of the grid wavelengths, to the controller 70,and then the controller 70, responding the instruction of the externalsystem, reads the initial parameters and the target values from thememory 60 corresponding to the target wavelength and sets thus fetchedinformation in a temporal memory implemented within the controller 70.The memory 60, as described above, stores the initial parameters and thetarget values linking with the wavelengths. However, the memoryimplemented within the controller 70 may temporarily hold the parametersand the values only for the target wavelength. The controller 70 readsthe initial parameters of the bias currents, I_(LD) and I_(SOA), thetemperature T_(LD), and the heater power, P_(Heater1) to P_(Heater3),and supplies the bias current I_(LD) to the SG-DFB region A through theelectrode 8 thereof.

Also, the controller 70 drives the TEC 31 so as to set a temperaturethereof equal to the initial temperature T_(LD), which also sets thetemperature of the t-LD 30 substantially equal to the initialtemperature T_(LD). Moreover, the controller 70 further provides thepower, P_(Heater1) to P_(Heater3), to the heaters 10 in the CSG-DBRregion B, which adjusts the reflection spectrum attributed to theCSG-DBR region B; and supplies the initial bias current I_(SOA) to theSOA region C. The temperature T_(LD) of the t-LD 30 and those of theheaters 10 may oscillate the t-LD 30 at a wavelength vicinity of thegrid wavelength, and the bias currents, I_(LD) and I_(SOA), may set themagnitude of the optical beam output from the t-LD 30 substantiallyequal to the target power; but the wavelength and the magnitude of theoptical beam do not always coincide with the target wavelength and thetarget power. Accordingly, the feedback control subsequently carried outmay set the wavelength and the magnitude to be equal to the gridwavelength and the target power.

The feedback control by the controller 70 is sometimes defined as theauto-power-control (APC) and the auto-frequency-control (AFC).Specifically, in the APC, the controller 70 adjusts the bias currentI_(SOA) supplied to the SOA region C such that the photocurrent I_(m1)sensed through the first PD 53 coincides with the target photocurrent,which means that the magnitude of the optical beam output from the t-LD30 coincides with the target power. On the other hand in the AFC, thecontroller 70 adjusts the temperature T_(LD) of the TEC 31 such that theratio of the photocurrents I_(m2)/I_(m1), where I_(m2) is thephotocurrent output from the second PD 56, coincides with the targetratio of the target wavelength. Thus, the wavelength and the magnitudeof the optical beam output from the t-LD 30 may be precisely adjusted inthe target wavelength and the target power at the target wavelength.

An algorithm of the AFC using the ratio of the photocurrentsI_(m2)/I_(m1) will be further precisely explained. FIG. 4 showsexemplary transmittance of an etalon, where the horizontal axis denotesa wavelength and the vertical axis corresponds to the transmittance. Thetransmittance of an etalon periodically varies as FIG. 3 shows. In thewavelength tunable system 100 applicable to a system following the ITU-Tstandard, an etalon with a period in the transmittance spectrum ofaround 100 GHz is preferably selected because the ITU-T standard for thewavelength division multiplexing (WDM) communication system defines aminimum span between two grids to be 50 GHz in a range of 192 to 197THz.

Because an etalon 54 has the transmittance shown in FIG. 4, taking theratio of optical beams outgoing from the etalon 54 against that incominginto the etalon, which corresponds to the transmittance, the wavelengthof the optical beam may be estimated. The former beam, outgoing from theetalon 54, may be detected through the second PD 56 as the photocurrentI_(m2), while, the former beam, incoming into the etalon 54, may bedetected through the first PD 53 as the photocurrent I_(m1). Thus, theratio of the two photocurrents gives the transmittance of the etalon,namely, the wavelength of the optical beam just incoming into the etalon54, and the feedback control of the AFC sets this ratio I_(m2)/I_(m1)equal to the transmittance of the etalon 54 at the target wavelength byadjusting the temperature of the TEC 31.

The ratio I_(m2)/I_(m1) in the target value thereof is preferablyintermediate between the maxima and the minima in the transmittance ofthe etalon 54, because midways give substantial slopes in thetransmittance, which may increase the closed loop gain of the AFC.While, when the target value of the ratio I_(m2)/I_(m1) is set close tothe maximum or the minimum in the transmittance where the variation ofthe transmittance against the wavelength becomes relatively small, thefeedback loop of the AFC becomes hard to set a substantial loop gain andan error range around the target grid wavelength expands.

As described, when the target wavelengths follow the grid wavelengthsdefined in the ITU-T, the wavelength tunable system 100 preferablyimplements an etalon 54 having a period in the transmittance thereofsubstantially equal to the span of the ITU-T grids. In such a case, theetalon 54 in the temperature thereof is unnecessary to be adjustedbecause once sets a temperature of the etalon 54 at one grid wavelength,wavelengths showing the transmittance of the etalon 54 same with thatonce determined by the AFC correspond to the grid wavelengths of theITU-T system. Thus, in order to drive the t-LD 30 exactly at the targetgrid wavelength, two procedures are inevitable where the t-LD 30 isfirstly driven by the initial parameters and secondly driven by AFCusing the target values.

Next, procedures using the wavelength monitor 80 to evaluate the initialparameters and the target values to be set within the memory 60 of thesystem 100 will be described. First, a procedure comparable to thepresent embodiment will be described as referring to FIG. 4, where FIG.4 indicates the grid wavelengths, Alto λ₉₆, and orders thereof whoseinitial parameters and target values are to be evaluated by numeralsabove the arrows. Broken lines indicate a wavelength range where thet-LD 30 may oscillate, or a wavelength range the ITU-T defines.

The procedure comparable to the present invention first drives the t-LD30 at a wavelength in an intermediate wavelength range, for instance,the grid wavelength λ₄₇, by supplying initial parameters to the t-LD,which are empirically obtained or estimated in advance to the proceduresdescribed herein. For instance, the empirical parameters may be obtainedfor another t-LD whose initial parameters are obtained just before thesubject t-LD. In another case, the empirical parameters are thoseobtained for another target wavelength just before the present targetwavelength. The empirical parameters include the bias currents, I_(LD)and I_(SOA), the temperature T_(LD) of the TEC 31, and the power,P_(Heater1) to P_(Heater3), for the heaters 10. The t-LD 30 is firstdriven by such empirical parameters. Then, the feedback control of theAFC sets the wavelength of the optical beam just output from the t-LD 30equal to the target wavelength by adjusting the temperature of the t-LD30 as monitoring the output beam by the wavelength monitor 80. When thewavelength of the optical beam currently output from the t-LD and outputpower thereof coincide with the target wavelength and the target power,the controller 70 stores the parameters and the target values of thephotocurrents, I_(m1) and I_(m2), in the memory implemented therein andtransfers those parameters and values to the memory 60 as linking withthe target wavelength.

Then, the evaluation iterates the procedures for obtaining the initialparameters and the target values using the wavelength monitor 80 for theshorter target wavelengths, λ₄₆ to λ₁, and stores those initialparameters and target values determined by the AFC and the APC at therespective target wavelengths, λ₄₆ to λ₁, in the external memory 60. Inthe AFC and the APC at one target wavelength, the empirical parametersfirst set in the t-LD 30 and the TEC 30 may refer to those determinedfor the target wavelength immediate before the current targetwavelength.

The comparable procedure thus described determines the initialparameters and the target values first for a target wavelength in acenter of the wavelength range, then, iterates the evaluation to theshorter wavelengths and finally from a center of the wavelength rangetoward the longer wavelengths. One of reasons why the procedurecomparable to the present invention begins at a center wavelength isthat the t-LD 30 may oscillate stably at center wavelength in thewavelength range. This reason should be understood by mechanismsdescribed below.

FIGS. 5A and 5B show the gain spectra in the SG-DFB region A, and themagnitudes of the optical beam output from the t-LD 30. The gainspectra, as illustrated in FIGS. 5A and 5B, show a convex behavior.Specifically, the optical gain in the SG-DFB region A becomes greater ina center of the wavelength range compared with those in peripheralregions. When the optical gain in the magnitude thereof is enough in theperipheral regions, even if the optical gain becomes smaller comparedwith those in the center of the wavelength range, the t-LD 30 may stableoperate even at the peripheral regions, and the initial parameters andthe target values may be obtained by the AFC and the APC, as shown inFIGS. 5A and 5B.

However, some of t-LDs show the optical gain that is insufficient for at-LD to operate stably. In such a case, as shown in FIGS. 6A and 6B, thet-LD may not operate stably at the peripheral wavelengths, for instance,λ₁ and λ₉₆, even when the system supplies the initial parameters thatare empirically obtained, or those obtained for a target wavelength justbefore the iteration of the measurement. FIG. 6A schematicallyillustrates a case where the t-LD 30 may be un-operable at both oflonger and shorter wavelengths in the wavelength range for a reducedoptical gain, while, FIG. 6B show another cases where the t-LD may notoperate at least one of the longer and shorter wavelengths. When a t-LDmay not stably operate, the feedback control of the AFC and the APC forobtaining the initial parameters and the target values becomesmeaningless no longer. Moreover, when such a situation occurs, namely, at-LD may not operate stably, the initial parameters and the targetvalues, which are obtained for the target wavelengths at which the t-LDmay operate stably, becomes useless because such a t-LD becomes unableto be delivered.

First Embodiment

Therefore, the present invention may provide an algorithm that may notwaste the initial parameters and the target values that are evaluatedfor the precedent grid wavelengths.

FIG. 7 shows a flow chart for evaluating the initial parameters and thetarget values first set for the t-LD 30 and the feedback loop of the AFCand the APC according to embodiment of the present invention; while,FIGS. 8A and 8B schematically illustrate an order of the targetwavelengths at which the evaluation of the parameters and the values arecarried out. FIGS. 8A and 8B denote the wavelengths in the horizontalaxis, while, the magnitudes of the optical beam output from the t-LD 30in the vertical axis thereof. Also, solid lines in FIGS. 8A and 8B,schematically indicates the optical gain attributed to the SG-DFB regionA, and numerals indicating the arrows corresponds to the order by whichthe evaluation of the parameters and the values are carried out.

Specifically, at step S10, the t-LD 30 is first driven by theparameters, which are empirically determined for the t-LD 30 with thearrangement shown in FIG. 2, for the shortest target wavelength λ₁ andfor the longest target wavelength λ₉₆. The parameters are the biascurrents, I_(LD) and I_(SOA) supplied to the SG-DFB region A and to theSOA region C, respectively; the temperature T_(LD) of the t-LD 30, thepower, P_(Heater1) to P_(Heater3) provided to the heaters 10, wherethose parameters may be common to all target wavelengths, λ₁ and λ₉₆.

Then, the system checks through the wavelength monitor 80 whether thet-LD 30 in the output wavelength thereof may be nearby shortest andlongest target wavelengths at step S12. When the t-LD 30 may generatethe optical beam, the laser light, which corresponds to “Yes” at stepS12, the system performs the feedback loops of the AFC and the APC suchthat the optical beam currently output from the t-LD 30 in thewavelength thereof coincides with the longest and shortest wavelengths,and the power thereof becomes equal to the target power. After theoptical beam currently output from the t-LD 30 coincides in thewavelength thereof and in the power thereof with the target wavelengthand the power, the system stores at step S14 the parameters currentlyset in the t-LD 30 and the TEC 31, and the photocurrents, I_(m1) andI_(m2), evaluated through the feedback loop of the AFC and the APC, asthe initial parameters and the target values at the shortest and thelongest wavelengths.

Thereafter, the system advances the procedures for evaluating theparameters and the values in relatively shorter target wavelengths,λ₂˜λ₄, and relatively longer target wavelengths, λ₉₃˜λ₉₅, by setting theempirically obtained parameters, confirming whether the t-LD 30generates an optical beam or not, and performing the feedback loop ofthe AFC and APC. In those steps, the system preferably performs theevaluation of the parameters and values alternately in the shortertarget wavelengths and the longer target wavelengths from the longer andshorter ones. That is, after the evaluation for the shortest and thelongest wavelengths, λ₁ and λ₉₆, the system preferably performs theevaluation at the wavelength λ₂ that is the shortest wavelength amongthe target wavelengths not performing the evaluation yet, then at thewavelength λ₉₅ that is the longest wavelength among the targetwavelengths not performing the evaluation yet, and so on.

When the procedures thus described once find a target wavelength atwhich the t-LD 30 may not generate an optical beam and the feedback loopof the AFC and the APC to get the parameters is unable to carry out,which corresponds to No at step S18, the system ceases the evaluation ofthe parameters and the values, and decides that a t-LD 30 currentlyevaluating the parameters thereof is an inferior product and ceases thesubsequent procedures, which efficiently saves a time to find a failuredevice. On the other hand, the t-LD 30 may generate an optical beam atrespective shorter and longer target wavelengths and the subsequentfeedback loop of the AFC and the APC may be carried out, whichcorresponds to “Yes” at step S18, the system stores the initialparameters and the target values evaluated through the feedback controlin the memory 60 as linking with the target wavelengths at step S20.

The system further advances the evaluation of the parameters and thevalues for the target wavelengths in a center region of the wavelengthrange at step S22. For those target wavelengths, the order of theevaluation becomes optional because most t-LD having the arrangementshown in FIG. 2 may stably generate an optical beam in thosewavelengths. The order of the evaluation may be carried out one by onefrom the shortest target wavelength not performing yet to a longertarget wavelength, from the longest target wavelength not performing yetto a shorter wavelength, or alternately in shorter target wavelengthsand longer target wavelengths from a center target wavelength.

The system confirms through the wavelength monitor 80 whether the t-LD30 may generate an optical beam at respective target wavelengths at stepS24. When the t-LD 30 generate the optical beam, which corresponds to“Yes” at step S24, the system obtains the initial parameters and thetarget values through the feedback control of the AFC and APC, andstores the information into the memory 60 as linking with the targetwavelength in step S26. On the other hand, the t-LD 30 may generate nooptical beam, which corresponds to “No” at step S24, the system ceasesthe evaluation and decides the t-LD 30 is an inferior product.

When the t-LD 30 may generate no optical beam at the shortest targetwavelength λ₁ or at the longest target wavelength λ₉₆, which correspondsto “No” at step S12, the system narrows the wavelength range as shown inFIG. 8B. That is, the system eliminates the shortest or longest targetwavelength and revises the shortest and longest target wavelength to beλ₂ and λ₉₅, respectively, at step S28. Then the procedures to evaluatethe parameters and the values are performed for the revised targetwavelengths from step S10. The steps, S10, S12, and S18, are iterateduntil t-LD 30 may generate an optical beam at the revised shortest andlongest target wavelengths.

Thus, according to embodiment, the system first evaluates the parametersand the values for the shortest and longest target wavelengths bysetting empirically determined parameters. When the t-LD may generate anoptical beam, the system performs the feedback control of the AFC andthe APC to evaluate the initial parameters and the target values andstores thus evaluated information in the memory 60 as linking with thetarget wavelengths. Then, the system continues the evaluation forrelatively shorter and longer target wavelengths by setting empiricallydetermining parameters in the t-LD 30, confirming whether the t-LD 30generates an optical beam or not, performing the feedback control of theAFC and the APC loop, and storing the revised parameters and the valuesin the memory 60. Because, a t-LD shows the optical gain thereofreducing in a peripheral wavelength range, namely, relatively shorterand longer wavelengths; the system may find a failure t-LD in an earlierstage of the evaluation.

Also, the evaluation described above changes the target wavelengthsalternately between a relatively shorter one and a relatively longerone. Accordingly, the system may find a t-LD with a failure in at leastone of a shorter target wavelength and a longer target wavelength in anearlier stage of the evaluation; and may reduce the possibility forwasting the parameters and the values obtained in advance of the currentevaluation.

Also, according to the embodiment thus described, when the may notconfirm that the t-LD 30 generates an optical beam at the shortestand/or the longest target wavelength even when the empirical parametersare set therein; the system may shift and/or narrower the wavelengthrange at which the t-LD 30 is necessary to generate an optical beam anditerate the evaluation of the driving parameters. Thus, even when thet-LD 30 is unable to operate in the widest wavelength range, the t-LD 30may be approved as a limited grade product. Accordingly,

When the system confirms that the t-LD 30 generates an optical beam at arevised wavelength range, the system may store information concerning tothe grade of the t-LD 30 in the memory 60. The memory 60 may secure aspace and an address for holding the grade of the t-LD in addition tothe space for holding the initial parameters and the target values ascorrelating with the target wavelengths. This arrangement that thememory holds the grade of the t-LD 30 makes the management of wavelengthtunable system 100 essentially simple compared with a scheme thatanother system that manage the wavelength tunable system 100. The memory60 stores the initial parameters and the target values correlating onlywith the target wavelengths determined by the grade that is also storedin the memory 60.

The embodiment of the t-LD 30 thus described provides the CSG-DBR regionB that prepares three heaters 10 therein. However, the t-LD 30 in thearrangement thereof is not restricted to this CSG-DBR region B. FIG. 9shows a cross section of a t-LD 30A having another arrangement differentfrom those aforementioned. That is, the CSG-DBR region Ba shown in FIG.9 provides only one heater 10 and segments whose optical lengths arecommon in the CSG-DBR region Ba but different from that in the SG-DFBregion A.

The t-LD 30A shown in FIG. 9 also shows the optical gain graduallydecreasing in peripheries of the wavelength range. Accordingly, theevaluation procedures for determining the initial parameters and thetarget values preferably begin for the target wavelengths in theperipheries thereof. The t-LD 30 having three heaters 10 in the CSG-DBRregion B shows the reduction in the optical gain thereof in theperipheries of the wavelength range that is greater than that shown bythe t-LD 30A having only one heater 10 in the CSG-DBR region Ba.Accordingly, the evaluation procedures for the t-LD 30 of the firstembodiment preferably begin with the shortest and the longest targetwavelengths, λ₁ and λ₉₆.

The first embodiment defines that the shorter wavelength range includesthree target wavelengths λ₂˜λ₄, while, the longer target wavelengthsincludes three target wavelengths λ₉₃˜λ₉₅. However, the invention is notrestricted to those arrangements. The shorter wavelength range mayinclude only one target wavelength λ₂ and the longer wavelength rangemay include only one target wavelength λ₉₅. Also, the shorter wavelengthrange may include four or more target wavelengths, and the longerwavelength range may include four or more target wavelengths. Theshorter wavelength range preferably includes the second shortest targetwavelength λ₂ and the longer wavelength range preferably includes thesecond longest target wavelength λ₉₅.

While particular embodiment of the present invention has been describedherein for purposes of illustration, many modifications and changes willbecome apparent to those skilled in the art. Accordingly, the appendedclaims are intended to encompass all such modifications and changes asfalling within the true spirit and scope of this invention.

I claim:
 1. A method of determining initial parameters and target valuesfor feedback loops of an auto-power control (APC) and anauto-frequency-control (AFC) for tuning a wavelength and power of anoptical beam output from a wavelength tunable system that implements awavelength tunable laser diode (t-LD) operable at a plurality of targetwavelengths within a wavelength range, the t-LD being attributed with anoptical gain with a maximum in a center of the wavelength range butgradually decreasing in peripheries in the wavelength range, the methodcomprising steps of: iterating an evaluation of initial parameters andtarget values at target wavelengths in a preset order; the evaluationincluding steps of, supplying parameters to the t-LD, the parametersbeing empirically determined, confirming whether the t-LD generates anoptical beam or not, when the t-LD stably generates the optical beam,carrying out feedback loops of the AFC and the APC for determining theinitial parameters and the target values that set the wavelength and thepower of the optical beam currently output from the t-LD equal to one ofthe wavelengths and power for the one of the wavelengths currentlyperformed, and storing the initial parameters and the target values in amemory as linking with the one of the target wavelengths currentlyperformed, and when the t-LD generates no optical beam or unstablyoperates, shifting the wavelength range so as to exclude the targetwavelength currently performed, wherein the preset order is alternatelyselected between a shortest target wavelength and a longest targetwavelength at which the evaluations are not performed yet.
 2. The methodof claim 1, wherein the t-LD has an arrangement of a sampled gratingdistributed feedback (SG-DFB) region, a chirped sampled grantingdistributed Bragg reflector (CSG-DBR) region, and a semiconductoroptical amplifier (SOA) regions, the SG-DFB region showing the opticalgain, the CSG-DBR region having a heater that modify a temperature ofthe CSG-DBR region, the t-LD being mounted on a thermo-electric-cooler(TEC) that varies a temperature of the t-LD, wherein the parametersempirically obtained includes a bias current to the SG-DFB region, powersuppled to the heater in the CSG-DBR region, and another bias currentsupplied to the TEC, and wherein the initial parameters determined bythe feedback loops are the bias current for the SG-DFB region, the powersupplied to the heater in the CSG-DBR region, the another bias currentsupplied to the TEC, and another bias current supplied to the SOAregion, and the target values are a first photocurrent generated by aphotodiode that detects the optical beam and a ratio of a secondphotocurrent generated by a second photodiode that detects the opticalbeam through an etalon against the first photocurrent.
 3. The method ofclaim 2, wherein the etalon has periodic transmittance whose intervalbetween neighbor peaks substantially coincides with an interval betweenneighbor target wavelengths.
 4. A method of ranking a wavelength tunablelaser (t-LD) that provides a sampled grating distributed feedback(SG-DFB) region, a chirped sampled grating distributed feedbackreflector (CSG-DBR) region, and a semiconductor optical amplifier (SOA)region, the t-LD being mounted on a thermo-electric cooler (TEC) thatvaries a temperature of the t-LD, the CSG-DBR region providing a heaterthat varies a temperature of the CSG-DBR region, the t-LD emitting laserlight whose wavelength being to be discretely tunable within awavelength range by setting parameters in the t-LD, the SG-DFB regionhaving an optical gain that shows a maximum in a center of thewavelength range and becomes relatively smaller in peripheries of thewavelength range, the method comprising steps of: driving the t-LDnearby a shortest wavelength and nearby a longest wavelength within thewavelength range by setting parameters that are empirically determined;confirming whether the t-LD generates an optical beam whose wavelengthis around the shortest wavelength and the longest wavelength in thewavelength range with and power is around a designed power, when thet-LD is unable to generate no optical beam at the shortest wavelengthand the longest wavelength with the predetermined power or unstablyoperate, degrading a rank of the t-LD, and iterating a step of drivingthe t-LD at a wavelength next to the shortest wavelength and next to thelongest wavelength within the wavelength range until the t-LD generatean optical beam stably, and when the t-LD is unable to emit laser lightat a preset wavelength in a center portion of the wavelength range,deciding the t-LD to be in failure.
 5. The method of claim 4, whereinthe step of driving the t-LD includes a step of providing bias currentsto the SG-DFB region and the SOA region, power to the heater, and acurrent to the TEC.